Tru-Path

Case Studies 2-A & 2-B:  iPhone / Compass 55 (compass app) – Birmingham, AL (test site) & Picayune, MS (test site)

Row, row, row your boat gently up / down the stream???

Purpose: This blog is created to help readers a) better understand electronic compass [smartphone or rangefinder] residual azimuth deviation errors b) quantify the errors, c) model the errors, d) compensate for [correct] the errors, and e) influence the app vendors to apply the correction method within the affected smartphone app. Basically, we need to know (accurately) whether to go up / down the stream (path) we are traveling on.

Background: Case Study 2-A

This portion of this post will present the test results of the iPhone Compass 55 app operated at the Birmingham, AL test site with a strong magnetic/electromagnetic field. Recall that a previous post (1-A) in this series presented the test results of the Vectronix PLRF25C rangefinder (with compass) operating in the Birmingham, AL test site with a strong magnetic/electromagnetic field.

This post presents test results of the iPhone Compass 55 app at two test sites:

• The Birmingham, AL (2-A) test site is set in an urban environment with strong magnetic/electromagnetic influences including:
  • An electric power distribution station for eight (8) townhomes
  • At least eight operating heating/cooling (heat pump) units of 3 to 4 ton capacity – aligned North/South within 50 feet of the test site (East side)
  • An active highway – aligned North/South within 150 feet of the test site (West side)

• The Picayune, MS (2-B) test site exists in a 20 acre cow pasture with minimal magnetic/electromagnetic influence (field). The cattle were curious (disruptive) about what was taking place in their pasture. Recall that the Picayune, MS test site is 264 miles southwest of the Birmingham, AL test site.

Three major issues at the forefront of these tests include:

• What is the impact (if any) of changing location (latitude change) on the effectiveness of the method for correcting residual compass (azimuth) deviation errors – for the iPhone Compass 55 app? Fact: The earth’s magnetic field strength changes with the geographic location of the observer.
• What is the impact (if any) of changing the operating environment (minimal magnetic/electromagnetic influence) on the effectiveness of the method for correcting residual compass (azimuth) deviation errors – for the iPhone Compass 55 app? Fact: Electronic compass results (azimuth readings) are impacted by environmental influences.
• What is the impact (if any) of not recalibrating the iPhone compass (at the Picayune, MS site) on the effectiveness of the method for correcting residual compass (azimuth) deviation errors – for the iPhone Compass 55 app? Fact: The author was entirely willing to accept any error that may result from the decision not to recalibrate the device being tested – relying on the capabilities of the compensation method to correct the potential azimuth errors.

The Test Results (2-A)

The data collection equipment and procedures used at test site A (Birmingham, AL) were identical to those used at test site B (Picayune, MS). Also, these data collection procedures were the same as used for testing the compass accuracy of the Vectronix PLRF25C laser rangefinder – refer to the earlier posts.

At both test sites, the reference direction (True North) was established using sun position – a correct, defendable, and independent reference direction.

• True North (reference direction) was established based on the sun position relative to each test site’s geographic location on the date/time of each test.
• The iPhone compass was set to indicate azimuths relative to True North.

Recall: A residual (after calibration) compass deviation error persists throughout the entire 360 degree range of measurement.  The following data table presents the collected azimuth data (yellow) and the associated azimuth deviation error (orange). The data collection operations with the iPhone Compass 55 app were performed using three iPhone orientations – Horizontal, Portrait, and Landscape.

Notice that the azimuth deviation (error) is different for each orientation of the iPhone.  The Compass 55 app has done its best – yet residual compass azimuth deviation (error) persist.

Preview:  As we proceeded with the each compass app (one after another), we could not help concluding that each app had its own internal proprietary processing procedure to provide its “best” result for each azimuth reading – yet each app’s azimuth deviation (error) values were unique. Also, each compass app had its own quirks relative to the data collection process. For instance, due to the sensitivity of some apps, the data collection process was executed as follows: using ONLY

• Slow, deliberate movements
• Clockwise rotations
• Left-to-right motions
• No jerky movements

It was interesting to contrast (graphically) the residual compass azimuth deviation errors across multiple apps – to determine the consistency (or lack thereof) between apps being executed on a single smartphone (iPhone).

• In general, the shapes of the iPhone compass app (error) curves were relatively consistent – the data was collected using the same iPhone, on the same day, and in the same location.
• The relative positions of each (Horizontal, Portrait, and Landscape) curve were relatively consistent.

The “modeled” deviation curves (derived from the residual deviation error) for the iPhone Compass 55 app are presented below.  The modeled residual azimuth deviation compensation curves are presented in two different formats to allow the reader to seriously consider the deviation error – as measured throughout the full 360 degree range of measurement.

Note (below) that the modeled deviation curves of each iPhone compass app retain a “similar” relative relationship (shape) with each other – with the curves of Compass 55 retaining the most consistent relative shape.  However, the magnitudes and directions of the deviation error may differ quite a bit – unique to the particular app being studied.

The compensated deviation errors (remaining azimuth errors after compensation) for the iPhone Compass 55 app are depicted below.  Again, two display formats are provided to strengthen the perceived impact of the compensation method.

The predicted deviation errors for the iPhone Compass 55 app are depicted in the following chart.  The predicted error curves are the negative of the deviation curves; and the compensation method proved quite effective.

Background: Case Study 2-B

This portion of this post will present the test results of the iPhone Compass 55 app operated at the Picayune, MS test site with a minimal magnetic/electromagnetic field. Recall that a previous post (1-B) in this series presented the test results of the Vectronix PLRF25C rangefinder (with compass) operating in the Picayune, MS test site with a minimal magnetic/electromagnetic field.

The chosen test site was located in the middle of a 20 acre cow pasture.  Additionally,

• The site was located (geographically) 264 miles SW from the location of test site A (Birmingham, AL) – thus dealing with the fact that the earth’s magnetic field strength changes with the geographic location of the observer.
• The iPhone’s electronic compass WAS NOT recalibrated at test site B.  The author was entirely willing to accept any error that may result from the decision not to recalibrate the devices being tested – relying on the capabilities of the compensation method to correct the potential azimuth errors. The reader should recall that the iPhone compass WAS recalibrated just prior to the data collection operations at the Birmingham, AL test site.

“If you are working in a cow pasture, be aware that inquisitive cows can slow down your production rate.” – as illustrated in the previous photo.

Recall: The residual compass deviation error persists throughout the entire 360 degree range of measurement.  The following data table presents the collected azimuth data collected (yellow) and the associated deviation error (orange) for three iPhone orientations (horizontal, portrait, and landscape). The compass deviation error data was collected in the same manner as all the other error data presented in this blog series.

Notice that the azimuth deviation (error) is different for each orientation of the iPhone.  The Compass 55 app has done its best – yet residual compass azimuth deviation (error) remains present.

The “modeled” deviation (error) compensation curves for the iPhone Compass 55 app are presented below.  Each model is presented in two different formats to allow the reader to seriously contrast each app – for each iPhone orientation.

The compensated deviation errors (remaining azimuth error) for the iPhone Compass 55 app were determined.  Again, two display formats are provided to strengthen the perceived contrast.

The predicted deviation errors for the iPhone Compass 55 app were analyzed. The predicted error curves are the negative of the deviation curves; and the compensation method proved quite effective in each case.

Now, we can assess the impact of the three issues identified in the “Background” portions of this post.

• What is the impact (if any) of changing location (latitude change) on the effectiveness of the method for correcting residual compass (azimuth) deviation errors? Response: Minimal
• What is the impact (if any) of changing the operating environment (minimal magnetic/electromagnetic influence) on the effectiveness of the method for correcting residual compass (azimuth) deviation errors? Response: Minimal
• What is the impact (if any) of not recalibrating the iPhone’s compass on the effectiveness of the method for correcting residual compass (azimuth) deviation errors? Response: Minimal

Recall that the iPhone Compass 55 app’s measured residual deviation (azimuth) errors were (quite adequately) compensated in each test.

The reader should note that the Vectronix rangefinder azimuth readings indicated a stable electronic compass and solid software processes; while the iPhone compass apps do not perform as well.  It seems apparent that the iPhone app vendors all have their own preferences for handling the data provided by the iPhone electronic compass; and some vendors have gone to great lengths in their attempt to provide a good azimuth reading – with not much success.  Some observations made during the author’s attempt to capture good azimuth readings from the iPhone apps are offered below.

• For certain apps, rotating the iPhone very slowly can result in NO change in the reported azimuth value.  Tapping the iPhone (after rotating with no change) can result in a sudden change to a new azimuth value – the apparent “correct” value.
• For certain apps, rotating the iPhone in an anticlockwise direction can give results that are slightly different from those obtained when using a clockwise rotation – repeatability not so good.
• For certain apps, avoid jerky motions – unexpected results may appear.
• For certain apps, the results provided change with time – even a few seconds can make a difference.
• For all apps evaluated, the reader should have a little patience in order to capture the best data possible.

In the upcoming blog posts, we will deal with other iPhone compass apps. As with the tests of the iPhone Compass 55 app, the same methods will be used to collect, analyze, and present the results. When this set of blog posts is complete, we will have dealt with the following iPhone compass apps.

• Compass 55
• Compass Deluxe
• SpyGlass
• Theodolite